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Propeller I/O input impedance — Parallax Forums

Propeller I/O input impedance

El PaisaEl Paisa Posts: 375
edited 2006-05-15 05:58 in Propeller 1
Does any one have an idea·what the Propeller input impedance I/O pins are?.
Both off and on states.

Not only the resistive component but the input capacitance as well?

I am looking for something like the thevenin or norton equivalent circuit.

This may have a critical effect if a large series resistance is used when connecting a 5V circuit.

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-05-14 02:56
    You should not connect to a 5v circuit directly; if your source input is 5v level then you should limit th current into the pin with a 1k - 10k resistor.

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    Jon Williams
    Applications Engineer, Parallax
  • cgraceycgracey Posts: 14,206
    edited 2006-05-14 05:28
    In input mode, the I/O pins have nearly infinite resistance and about 6pf of capacitance. There are clamp diodes that limit pin voltage·from·VSS-0.4V·to VDD+0.4V.
    El Paisa said...
    Does any one have an idea·what the Propeller input impedance I/O pins are?.
    Both off and on states.

    Not only the resistive component but the input capacitance as well?

    I am looking for something like the thevenin or norton equivalent circuit.

    This may have a critical effect if a large series resistance is used when connecting a 5V circuit.
    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔


    Chip Gracey
    Parallax, Inc.
  • El PaisaEl Paisa Posts: 375
    edited 2006-05-15 01:48
    My only concern is the time constant introduced to the input.

    With a 10k resistor and a 6pf input capacitor, the time constant is 0.06 microseconds not to small compared with a clock frequency of 80Mhz (0.125 microseconds).

    Now with a series resistor of 100k and input capacitance of 6pf, the time constant is now 0.6 microseconds.

    For some high frequency applications specially video, this time constant will introduce a considerable delay.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-05-15 05:58
    Consider the solutions in 'The Art of Electronics' for going from +5 to +3.3. There are specific conversion chips available. Refer to Chapter Nine.

    The clamping diodes appear to have only so much capacity to bleed off the excess voltage. There is a thread [noparse][[/noparse]about May 5th] that discussed this and provided some data. 100k might work, if the frequency is appropriate; but how can one be sure?

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
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